In a statement, the Directorate General of Civil Aviation said the new aircraft "may be operated on the same schedule" as was operated by the plane grounded on the account of the new induction.
IndiGo has to replace the faulty engines on its entire A320neo fleet by January 31, 2020.
"During the review meeting on the issue, it was noted that efforts undertaken by the operator to replace all unmodified engines on their fleet by January 31, 2020, do not instil enough confidence with regard to the timely completion of the said task," the DGCA said in a statement.
"If left unaddressed, we may find ourselves in a situations, in which, we remain saddled with large number of aircraft with unmodified engines and operating on a schedule approved by us and we are left with the only option of i.e. to ground them on or after January 31st, 2020."
The regulator said that at this stage grounding the entire A320neo fleet will lead to a massive disruption and is a "sub optional solution".
"Though the DGCA does not wish to interfere with the import of aircraft, however, it is the considered view of the DGCA that from now onwards, every aircraft that is added to the existing fleet, should lead to one of those with unmodified engines to be grounded and the new aircraft may be operated on a new schedule as was being operated by the aircraft, which will be grounded," the statement said.
"DGCA perceives that this substitution of an aircraft with unmodified engines with modified one, will not cause any disruption and shall improve the safety of aircraft operations and maintain the regularity of operations."
( With inputs from IANS )